Preheat oven to 400°. Quarter beets lengthwise. Slice half lengthwise and half crosswise into wedges about 3/4 in. thick. Cut slab bacon into 1/2-in. cubes, or sliced bacon into 1 1/2-in. pieces.
In a shallow 3- to 3 1/2-qt. baking dish, toss beets, bacon, garlic, oil, and salt.
Bake, stirring every 15 minutes or so, until beets are tender when pierced and bacon begins to brown, 1 1/4 to 1 3/4 hours. Stir in chestnuts.
*Find slab bacon at well-stocked grocery stores or butcher shops. Soak garlic cloves in cool water 15 minutes to make peeling easier. Find peeled chestnuts at well-stocked grocery stores, or see how to peel them at sunset.com/chestnuts.